The present invention relates generally to optical communication systems and more particularly to optical attenuators.
Optical communication systems typically include a variety of optical fiber-coupled devices (e. g., light sources, photodetectors, switches, attenuators, amplifiers, and filters). The optical fiber-coupled devices transmit optical signals in the optical communications systems. Some optical signals that are transmitted in such optical communications systems have many different wavelengths (frequencies). Digital or analog data is transmitted on the different wavelengths of the optical signals.
Many optical communication systems are lossy in that the optical fibers used therein scatter (or absorb) portions of the optical signals transmitted therealong (about 0.1-0.2 dB/km). When portions of the optical signals transmitted on the optical fibers are scattered (or absorbed), the power associated with such optical signals is reduced. To compensate for power reductions attributable to the optical fibers, optical amplifiers are positioned in the optical communication system. The optical amplifiers increase the power of the optical signals transmitted along the optical fibers.
After the optical signals propagating along the optical fiber experience multiple cycles of power losses followed by amplification, power variations between the different wavelengths of the optical signals potentially occur. If not corrected, these power variations may cause adjacent wavelengths to interfere (cross-talk) with each other. Interference between adjacent wavelengths of the optical signals is a potential source of transmission errors.
Typically, optical attenuators are used to control the power variations between the different optical signal wavelengths in optical communication systems. Some optical attenuators control the power variations between the different optical signal wavelengths by reflecting portions of specified optical signal wavelengths provided thereto.
Many optical attenuators include a plate attached to a substrate with torsional members (e. g., rods, springs). The plate is coated with a reflective material. The plate is moveable relative to the substrate by applying a torque to the torsional members. The movement of the plate attenuates optical signals provided thereto by reflecting portions thereof away from the transmission path of the optical communication system.
One problem with optical attenuators that include reflective plates relates to their insertion loss. Optical attenuators, in an xe2x80x9coffxe2x80x9d state, typically reflect optical signals with near zero attenuation. Near zero attenuation in the xe2x80x9coffxe2x80x9d state requires that the reflective plates have very flat surfaces. Reflective plates with very flat surfaces are difficult to fabricate.
Also, near zero attenuation in the xe2x80x9coffxe2x80x9d state requires that the plane of the reflective plate be positioned parallel to the substrate. However, for a torsional plate structure, the torsional members are fragile such that the equilibrium rotation of the reflective plate potentially drifts after each xe2x80x9con/offxe2x80x9d cycle. Such drifting of the reflective plate affects its position plate relative to the substrate.
Thus, optical attenuators continue to be sought.
The present invention is directed to an optical attenuator having a structure in which a membrane covers a cavity formed on a substrate. The membrane is movable relative to the bottom surface of the cavity and constitutes a deformable mirror. Movement of the membrane relative to the bottom surface of the cavity attenuates optical signals impinging on the surface thereof. The movement of the membrane relative to the bottom surface of the cavity attenuates impinging optical signals by controlling the angle at which optical signals impinging off-axis are reflected from the membrane surface.
The membrane covering the cavity formed on the substrate is movable relative to the bottom surface of the cavity in response to an electrostatic field. The electrostatic field is generated by applying a bias voltage between the membrane and the bottom surface of the cavity formed on the substrate. The magnitude of the electrostatic field depends on the amount of the applied bias voltage.
Both the membrane and the substrate are preferably conductive so that the bias voltage may be applied across them to generate the electrostatic field. When either of the membrane or the substrate are insufficiently conductive to generate the electrostatic field, conductive layers are optionally formed on regions thereof. Such conductive layers are preferably formed on an overlying region of the membrane covering the cavity and on the bottom surface of the cavity formed in the substrate.
The substrate is made of a material typically used for integrated circuit fabrication. Examples of suitable substrate materials include silicon and quartz.
The membrane is made of one or more layers of material having physical properties (e.g., tensile strength, elastic properties, layer thickness) which permit membrane movement in response to the electrostatic field. Examples of suitable membrane materials include polysilicon, silicon nitride, silicon dioxide, and metals (e. g., gold).
The reflective properties of membranes made of insulating materials (e. g., silicon nitride) are enhanced with a layer of metal formed on the surface thereof. The metal layer would also constitute an electrode on the membrane.
The membrane optionally includes pores. The pores permit the removal of air in the cavity, facilitating movement of the membrane relative to the bottom surface of the cavity in response to the electrostatic field.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and do not serve to limit the invention, for which reference should be made to the appended claims.